A) Field of the Invention
The present invention relates to a semiconductor light emitting device and its manufacture.
B) Description of the Related Art
A semiconductor light emitting device is generally formed by a light emitting diode structure formed on a semiconductor substrate of GaAs, InP or the like. Light emitted in a light emitting layer propagates in an omnidirection. If the substrate is absorptive relative to light emission, light directed to the substrate is absorbed and attenuated. In order to efficiently direct emitted light to an external, such a structure is preferable that prevents light absorption in the substrate.
If a high reflectance plane is inserted between a light emitting layer and a substrate, light directed to the substrate is reflected and can be lead to an external. However, if a high reflectance Al layer or Ag layer is formed on the substrate, it becomes very difficult to epitaxially grow a semiconductor light emitting layer on the high reflectance layer.
A semiconductor light emitting device has been proposed which has such a structure that a semiconductor light emitting layer is bonded a conductive substrate through a high reflectance layer (e.g., Japanese Patent Laid-Open Publication No. 2001-189490 which is incorporated herein by reference).
FIGS. 4A to 4C are cross sectional views illustrating an LED manufacture method proposed in the above-mentioned Publication.
As shown in FIG. 4A, on a tentative or temporary substrate 42 of GaAs or InP, an LED structure 41 of a pn or np junction is grown. The LED structure may be one of various structures such as a homo pn junction structure, a hetero pn junction structure and a double hetero structure. A metal adhesive layer 43 is formed on a permanent substrate 44, made of a material having a high thermal conductivity, such as silicon, GaAs and alumina, the mental adhesive layer functioning as a reflection mirror. The material of the metal adhesive layer is selected from a group consisting of indium, tin, aluminum, gold, platinum, titanium, zinc, silver, palladium, gold-beryllium alloy, gold-germanium-nickel alloy and zinc-tin alloy. The LED structure 41 is bonded to the metal adhesive layer 43 in water, atmospheric air or alcohol and heat treatment is performed.
As shown in FIG. 4B, the tentative substrate 42 is removed by mechanical polishing or chemical etching. Etchant can be made of hydrochloric acid and phosphoric acid. An etch stopper of, for example, InGaP or AlGaAs, may be formed between the tentative substrate 42 and LED structure 41.
As shown in FIG. 4C, predetermined areas of the LED region are exposed and ohmic contact electrodes 411 and 412 are formed. If the metal adhesive layer is made of the same material as that of the ohmic electrode 411, such as gold-beryllium alloy, the metal adhesive layer may be used as the ohmic electrode 411 by etching the LED structure to the metal adhesive layer 43.
Light emitted from the LED structure 41 and directed toward the permanent substrate 44 is reflected at the metal adhesive layer 43 and transmits again through the LED structure 41 to be output to the external. In this manner, an external light emission efficiency can be improved.
If AuZn is used as the material of the metal adhesive layer 43 and ohmic electrode 411, Zn may be diffused into semiconductor so that it becomes difficult to realize an ohmic contact.
Good reflection characteristics are rather incompatible with good ohmic contact. An alloying process is necessary for forming ohmic contact. Morphology of an interface between semiconductor and metal alloy at the ohmic contact may become rough or metal may diffuse to lower the reflectance.
Solder or eutectic may be used for bonding two substrates. In this case, if solder or eutectic impregnates the reflection layer, the reflection characteristics of the reflection layer are degraded. When two substrates are bonded together, solder or eutectic may cause ball-up.